Coherence of Supercontinuum Light

Supercontinuum (SC) light fields are characterized by broadband spectra that may extend from ultraviolet to near-infrared. Such SC fields can be generated by nonlinear processes in solid-state bulk media and liquids, or in optical fibers. In the latter case, SC pulse trains with high spatial coherence can be produced, which however may feature widely different spectral and temporal properties depending on the excitation conditions. In this review, we consider the coherence properties of SC pulses using the second-order coherence theory of nonstationary light. We begin with an overview of SC generation and explain how different nonlinear processes give rise to variable coherence properties. After a brief review of the coherence theory of nonstationary light, we show how second-order correlation functions in both time and frequency domains can be constructed from numerical simulations of ensembles of individual pulses. Two types of convenient modal representations of these correlation functions are presented, and a connection to the widely used "first-order" spectral coherence function is made. We then consider several interferometric techniques that have been used to measure this first-order coherence function, and discuss methods for experimental determination of the second-order correlations. Finally, some conclusions are drawn and certain as-yet unresolved questions regarding the coherence of SC are outlined.